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It looks like the technology half-life for these things is ~2 years,

I disagree with this.  I have had two GPS units: first a garmin 12map then a Garmin 60C.  If the 12map had not died, its capabilities would be serving me well today and it would be 10 years old.  The 60C gave me color and some bells and whistles, but nothing vital.

My 60C is around 6 years old now.  I could go for an improved model, but each time I have looked, the improvements were what I considered to be quite minor.